I know bigger is probably better. I'm wiring the dual batteries in the LX and have some 4g lying around. Is that sufficient? or should I just go the 2g route and be done with it? I like the idea of saving a couple bucks on the 4g as I have enough to do about half the job.

I'll eventually have all the things on the LX (ham, aprs, fridge, cb, lights, winch, etc...)

right now I have the solenoid right next to the snd tray on the firewall...so to round up, lets say 10' (+) from primary battery to the solenoid as well as 10' (-) to from primary to 2nd battery. then a short jump of about 1-2' from 2nd battery to solenoid.

Depends on length of run and what you're hanging off of it. For charging and those loads you list 4 AWG cable is probably sufficient.

2AWG us about 0.00051 ohms per foot
4AWG is about 0.00082 ohms per foot

So if the current on the cable is 50 amps and the run is 20 feet, you can find the voltage drop (0.82V for #4 and 0.51V for #2). Significant but not earth shattering.

If you're charging a very flat house battery and all the charging current can go to it (e.g. the starting battery is either switched out or not drawing any), then #2 will start to look more necessary. IOW, 100A at 20' gives you 1.64V vs. 1.02V. That means if the forcing voltage is 14.4V but the battery is seeing 12.76V compared to 13.38V. The difference here is that the battery won't actually charge much at 12.8V while it will at 13.4V.

The larger issue is in this example the #4 would dissipate 164W while the #2 dissipates 102 watts. That's a sizable difference in how warm the cable will get.

Thanks for the excellent explanation! That makes perfect sense! (and maybe helps me determine the V-drop on the 60!) I think I'll do the short run(s) with the 4g and go with 2g for the longer run to the solenoid. Thanks!!

Yup, you should keep all runs as short as possible. Also going larger is never a negative on performance, just practical (larger = harder to run, larger terminals) and financial.

If you have #4 and the criteria are met, no reason to spend the money buying new cable. I'd probably use what I had, use fuses maybe and be mindful of the isolator and system to make sure I don't exceed what it can do practically. In the end how often will the max current situation happen? If it's often, then you need to be more concerned than if it might never or once a year or two.

Also would depend to some extent how the voltage is regulated. Your charging system uses battery voltage to adjust itself, so it's not impossible to compensate for the voltage drop by letting the regulator creep up. That might cause other issues (like your other battery and all the other devices in the chain seeing high voltage). But it's one maybe solution. Another simple one is to watch the discharge on the batteries and periodically start the truck to keep it from going super flat so you never get to the point that the house battery consumes very high current for long periods.

In the real world, almost no one gets their performance out of really large cable because few people know how to make proper connections. Connections is where all the power is lost, not the wire itself.

When I am building cable for winches and batteries stuff (like 50kW LASER power supplies), what I do is buy those big-ass honkin' terminals that have a cup to stick the wire into. The cup should be as close to the same diameter of the cable as is practical for the sizes they have. Then I stick the wire in and fill it up with molten solder.

It takes a lot of practice to do this right without buggering up the end of the insulation of the cable and also not get a cold solder joint, but that's the only way to get your money's worth out of thick gauge wire. The trick is to preheat the terminal with a torch, and then fill all that space with solder as quick as you can. Then the very moment you have the cup filled and all the spaces between the strands filled, remove the heat and get a heatsink clamped onto the cup so the wire insulation doesn't burn too bad. I normally have some heatshrink on there too; when things cool down enough slip the heatshrink over the end and then shrink it down to keep the strands sealed from the outside world.

Great point...I'll have to practice a little. I have definitely wrecked the insulation on some larger guage wire trying properly heat it for a good solder joint. I just picked up a 200/300W soldering gun - that 30W thing is useless unless you are solder 22g wire.